The "undefined" in this sentence doesn't refer to the existence of the property, it refers to the valueundefined. In JavaScript, when accessing a property doesn't exist, accessing it will return the value undefined:

foo.bar
// undefined

There are other situations where undefined is created, e.g. when you define a variable without an initial value:

var abc;
abc;
// undefined

Knowing that we can test what happens when we directly access a property on undefined: